1. Field of the Invention:
This invention relates to array antennas and more particularly to the formation of multiple beams with independent control circuitry.
2. Description of the Prior Art:
In U.S. Pat. No. 4,318,107 which issued on Mar. 2, 1982 to R. Pierrot et al. a microstrip monopulse antenna is described for providing independent sum and difference channels. The microstrip antenna as shown in FIG. 2 of `107 includes a plurality of microstrip radiating elements or "pads", a feeding/receiving circuit and a connecting means for connecting the feeding circuit to a predetermined feed point of each radiating element. A central microstrip radiating element provides a sum channel and at least one pair of radiating elements positioned symmetrically with respect to the central radiating element provides a difference channel. The respective feed points of the radiating elements have a predetermined eccentricity with respect to the zero field radio center of the radiating element in the axis of polarization defined by the eccentricity of the feedpoint of the central radiating element.
In U.S. Pat. No. 4,316,192 which issued on Feb. 16, 1982 to J. H. Acoraci. a beam forming network is shown in FIGS. 3 and 7 for providing sum and difference patterns having omnidirectional sidelobes. The beam forming network is coupled through phase shifters for steering the beam and through a Butler matrix for adapting the beam to a circular array antenna. The circular array antenna may include 64 dipole elements where eight columns of 8 dipole elements each are equally spaced around a metal cylinder which comprises the ground plane. The cylinder may be 5" in diameter.
In U.S. Pat. No. 4,128,839 which issued on Dec. 5, 1978 to A. D. McComas, a circular array antenna is shown in FIG. 3 consisting of 8 monopoles mounted above a ground plane with an upstanding portion forming a cylinder internal of the 8 dipoles which acts as a reflector. In FIG. 4 the monopoles are shown coupled through an 8 port Butler matrix to phase shifters which in turn are coupled to a passive beam forming network which may, for example, form a sum and difference pattern.
In a publication entitled "New High-Frequency Antenna: The Passive Network Array" by J. H. Dunlavy, Jr. Electronics, Jan. 3, 1964, pages 32-36, an end-fired coupled array consisting of two closely spaced end loaded dipole elements is described with currents of equal amplitude having a relative phase difference equal to 180.degree. minus the dipole spacing in electrical degrees. In FIG. 2, on page 35, the minimum front-to-back ratio measured at 10 MHz frequency is approximately 15 db.
It is known in the art by those practicing antenna design that a flat microstrip or patch dipole antenna arranged parallel to and in close spaced relationship with a ground plane conductor will exhibit a broad side antenna pattern, that is, a generally hemispheric antenna pattern on the dipole side of the ground plane with the ground plane forming the flat side of the hemisphere. If, however, two such patch dipoles, for example, are each arranged in the same close spaced relationship with and parallel to a ground plane conductor, separated from one another by a quarter wavelength of their operating frequency and have their feed points connected through a quarter wave phase delay, the two dipoles will form an end firing antenna element, whose antenna pattern will be directed generally along a line connecting common points on the dipoles and in the direction of phased delay.
In a publication entitled "Promising Array Developed, Successfully Tested, Then Dropped" by C. D. LaFond, Missiles and Rockets, Mar. 9, 1964, pages 33-35, a multiple-beam cylindrical array is described which makes possible for the formation of simultaneous multiple beams from a cylindrical array through the use of lossless passive transmission line networks. When the cylindrical array is used for multiple beam output, the antenna elements of the cylindrical array are excited by respective isolated network inputs. Each network input is associated with a beam in a specific direction and all beams are dispersed symmetrically throughout the 360.degree. azimuth angle.
It is therefore desirable to provide two beam forming networks for generating antenna patterns wherein one beam forming network is coupled to an array of antenna elements for radiating an antenna pattern in a forward direction and a second beam forming network is coupled to the same array of antenna elements for generating an antenna pattern in a reverse direction.
It is further desirable to provide an array of antenna elements subdivided into a plurality of subarrays, wherein each subarray comprises two radiating elements.
It is further desirable to provide a low profile antenna wherein two antenna elements comprise a subarray for radiating microwave energy in the forward direction in response to a first signal and for radiating energy in a reverse direction in response to a second signal.
It is further desirable to provide a plurality of subarrays arranged in a circle on a radius to provide a low profile circular array.
It is further desirable to provide a plurality of subarrays positioned along a path wherein the path could be along a flat, cylindrical, spherical or conical surface for radiating energy in a first direction in response to a first input and a second direction in response to a second input.